Porosity tester for coating materials



Oct. 23, 1951 1'. J. CONNOR v 2,572,597

POROSITY TESTER FOR COATING MATERIALS Filed Jan. 2. 195] I Inventor:Thomas J. Connor,

-l is Attorney.

Patented Oct. 23, 1951 POROSITY TESTER FOR COATIhlG MATERIALS Thomas J.Connor, Lower Merlon, Pa., asslgnor to General Electric Company, acorporation of New York Application January 2, 1951, Serial No. 204,013

1 Claim. 1

This invention relates to apparatus for testing coating materials andmore particularly to apparatus for determining porosity innon-conducting coating materials applied on metallic surfaces.

The common method of protecting exposed metallic surfaces is to coverthem with a thin layer of a non-conducting coating material such aspaint, lacquer or the like. Where a metallic surface is to be exposed tothe atmosphere, as for example are many transformer and circuit breakertanks, it is especially important that the protecting layer becontinuous over the entire surface. Even small holes or pores in thecoating layer, imperceptible to the human eye, may lead to flaking ofthe layer and the forming of rust and corrosion on the metallic surface.

In the past, the detection of these small pores in the layer of coatingmaterial has presented a serious problem. The means principally employedfor determining the condition of the layer of coating material has beenvisual inspection, but unfortunately, visual inspection, no matter howcarefully performed is often subject to serious error. Layers of coatingmaterial which appear to the eye to be continuous throughout may in factcontain a myriad number of extremely small holes or pores.

It is an object, therefore, of this invention to provide new andimproved means for determining porosity in non-conducting coatingmaterial applied on metallic surfaces.

It is another object of this inventionto provide a new and improvedportable device for determining porosity in non-conducting coatingmaterial applied on metallic surfaces, which harms neither the layer ofcoating material nor the metallic surface by its operation.

In this invention, the metallic structure, on whose surface lies thenon-conducting coating layer for which a porosity test is desired, isconnected in open series circuit with a source of electrical potential,a current indicating device, and a metallic member or plate. Themetallic plate is positioned to contact a pad of absorbent materialsaturated with .a water solution of a volatile salt, such as amoniumcarbonate. The metallic plate, the absorbent pad, anda supply of thesolution for continuously wetting the pad are ordinarily contained in aportable applicator member. To test for porosity in the coating layer,the pad is rubbed over the coating layer so that the solution containedin the pad permeates any pores in the coating layer. The contact of theelectrically conductive solution with any bare metal of the metallicsurface completes the circuit through the indicating device. Theindicating device by its deflection provides an indication of thepresence of the pores in the coating layer.

For a better and more complete understanding of my invention, togetherwith additional ob jects and advantages thereof, reference should now behad to the following description and accompanying drawing which is aschematic diagram of a preferred embodiment of this invention.

Referring to the drawing, a metal sheet I has a layer 2 ofnon-conducting coating material, such as paint, applied over the greaterportion of one of its surfaces. An alligator clip 3 by gripping sheet Iin an area which is not covered by layer 2 provides a good electricalconnection to the sheet. Clip 3 is connected by a wire 4 to an outputterminal 5 of a portable battery and meter case 6.

In case 6, terminal 5 is serially connected with two dry cell batteries1 and la, an indicating device 8, such as a direct current ammeter, andan output lead 9. Lead 9 is connected through wire it to a terminal llmounted on the closed end of a portable applicator member l2. Applicatormember I2 is shown in the drawing as a tubular container having a closedend and an open end. The shape of member I2 is not essential to theoperation of the device, but whatever shape is employed, it must form acontainer having at leastone opening to the space enclosed within it.

Positioned within member I2 is a member or plate l3 which is made of anelectrically conducting metal such as silver. In the preferredembodiment of this invention illustrated in the diagram, plate [3 ismovable within member l2. However, plate l3 may be positionedpermanently by screws or other means without appreciably affecting thefunctioning of the device, so long as the positioning means used do notseal off the space on one side of plate I3 from the space on the otherside. Filling the open end of member [2 and contacting plate I3 is a padI formed of absorbent material, such as felt. Pad I4 is held in place byfriction with the inner wall of member It. In order that sumcientfriction be provided to hold pad I securely, pad I4 is made slightlyoversize with respect to the opening in member l2 and is forced into theopening. Plate I3 is so positioned within member I! that, when pad Itcomes to firm contact with the plate, a portion of pad It stillprotrudes be- 55 yond extremity of member l2.

A spring I5 provides an electrical connection between terminal I I andplate I3, and, when plate I3 is movable, also serves to keep plate I3 infirm contact with pad I4. The space surrounding spring I5 is filled withan electrically conducting water solution I6 of a volatile salt, such asammonium carbonate. Since plate I3 does not seal off the spacesurrounding spring I5, pad I4 is continuously wetted by solution I6.

In operation applicator member I2 is moved so that the exposed portionof pad I4 is rubbed on layer 2 over all the area to be tested forporosity. If any pores exist in layer 2, as pad I4 is rubbed over thesepores, solution I6 penetrates them and contacts the metal surface ofsheet I. Since salt solutions in water, such as the ammonium carbonatesolution I6, are electrically conductive, the contact of the solutionwith the metal completes the electrical circuit through indicatingdevice 8. The wetted felt pad I4 between plate I3 and metal sheet I inefiect forms an electrolytic cell for the passage of current. Thecurrent indicated by device 8 depends upon the area of metal contactedby solution I6, i. e. the number of holes in or the porosity of layer 2.Thus this invention not only provides means for determining the presenceof porosity in a layer of coating material, but also provides a relativemeasurement of the seriousness of the porosity.

Solution I6 may be formed of almost any salt which does not leave acorrosive residue that will interfere with the durability of the coatinglayer. It has been found that a water solution of ammonium carbonategives excellent results. salt film deposited on the coating layer by asolution made of ammonium carbonate decomposes after a few hours intothe vapors of carbon dioxide, ammonia and water. Examples of other saltswhich may be used to form solutions suitable for employment with thisinvention are ammonium carbamate, ammonium carbamate acid carbonate,ammonium formate, ammonium bicarbonate, ammonium cyanate, ammoniumfluoroborate and ammonium thiocarbonate. Preferably, the salt usedshould be one possessing fairly high solubility as well as highionization. In some cases it is desirable to add a small amount ofwetting agent, such as aerosol, to the solution. The purpose of thewetting agent is to aid in the solution in permeating very small holesor pores in the coating layer.

Plate I3 may be perforated to allow better flow of solution I6 into padI4, particularly when the plate is permanently positioned. Theperforations The 4 not only aid the flow of solution but also help toeliminate polarization of the plate.

This invention possesses a major advantage in that, since it damagesneither the metallic surface or the coating layer, the porosity test maybe made on the actual surfaces which are to go into use rather thanbeing made on a test sample. Moreover, the invention is not restrictedto metallic surfaces small in area but may be used equally as well withlarge surfaces, such as for example the outside of power transformertanks.

Although in accordance with the provisions of the patent statutes, thisinvention has been described as embodied in concrete form, it should beunderstood that the invention is not limited to the precise structure orexact connections shown in the drawing since these are merelyillustrative and modifications and alterations willreadily suggestthemselves to persons skilled in the art without departing from the truespirit of this invention or from the scope of the annexed claim.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

A device for determining porosity in a layer of non-conducting coatingmaterial applied to a metallic surface comprising a portable applicatormember including a container having an open end; a metallic platepositioned within said container; a pad of absorbent material contactingsaid plate, filling said open end, and protruding beyond the extremityof said member; an electrically conductive water solution of a volatilesalt filling said container and wetting said pad to establish electricalcontact between said plate and said metallic surface through any poresin said coating material in response to movement of said pad across saidcoating material; and a circuit arranged to be completed by said contactincluding a source of voltage and a current indicating device, saidcontact to cause a deflection on said indicating device indicative ofporosity in said layer of coating material in response to permeation ofsaid solution into the pores in said coating material.

THOMAS J. CONNOR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Wright May 2, 1950

